Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications!

Dehumidification - Removing Moisture from Air

Dehumidifying by cooling, adsorption or absorption.

Air may be dehumidified - moisture or water removed - by

  1. Cooling - condensation of vapor
  2. Adsorption of water vapor
  3. Absorption of water vapor

1. Cooling the air - vapor condensation

In a cooling system humidity is reduced by cooling the air below the dew point. A part of the moisture in the air is condensed and drained out.

The cooling and dehumidification process in Mollier diagram:

Mollier diagram - cooling and dehumidification

2. Adsorption

In an adsorption system humidity is reduced with an adsorbent material like silica gel or activated alumina.

Adsorption is a physical process in where

  • moisture is condensed and held on the surface of the material

without any change of in the physical or chemical structure of the material. The adsorbent material can be reactivated by heat.

  • Temperature for reactivation: 160 - 170 oC
  • Heat required for reactivation: 4800 - 4800 kJ/kg water removed

Silica gel - SiO2

Silica gel - SiO2 - is a hard, adsorbent, crystalline substance and very porous. Voids are about 50 - 70% by volume and adsorbs water up to 40% of its own mass. The bulk density of silica gel is 480 - 720 kg/m3. The specific heat is 1.13 kJ/kgK.

Activated alumina

Activated alumina is about 90% aluminum oxide Al2O3 and very porous. Voids are about 50 - 70% by volume and adsorbs water up to 60% of its own mass. The bulk density is 800 - 870 kg/m3. The specific heat is 1.0 kJ/kgK.

3. Absorption

In an absorption system humidity is reduced with an absorbent material such as a calcium chloride solution.

Absorption involves a

  • change in the physical or chemical structure of the material

In general - it is not easy to reactivate the material.

Related Topics

  • HVAC Systems

    Design and sizing of Heating, Ventilation and Air Conditioning systems.

Related Documents


Search is the most efficient way to navigate the Engineering ToolBox.

Engineering ToolBox - SketchUp Extension - Online 3D modeling!

3D Engineering ToolBox Extension to SketchUp - add parametric components to your SketchUp model

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with older versions of the amazing SketchUp Make and the newer "up to date" SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp Make/Pro from the Extension Warehouse !

Translate this Page

Translate this page to Your Own Language .

About the Engineering ToolBox!

Privacy Policy

We don't collect information from our users. More about

We use a third-party to provide monetization technologies for our site. You can review their privacy and cookie policy here.

You can change your privacy settings by clicking the following button: .


This page can be cited as

  • The Engineering ToolBox (2003). Dehumidification - Removing Moisture from Air. [online] Available at: [Accessed Day Month Year].

Modify the access date according your visit.

3D Engineering ToolBox - draw and model technical applications! 2D Engineering ToolBox - create and share online diagram drawing templates! Engineering ToolBox Apps - mobile online and offline engineering applications!

Unit Converter